436
INJECTION MOLDS FOR THERMOPLASTICS
PLASTICS MOLD ENQlNEERlNQ HANDBOOK
be cooled or heated, and it will also tell the desired temperature range. Molds that run on several materials may need provision for heating and cooling, : Paradoxically, when the material is being "cooled" above the ambient temperature, it may be necessary to include heater elements that will facilitate cooling down from the melt temperature to the "set" temperature. High temperature liquid coolants such as hot water, steam, Dow Therm, oil, etc., are also used to achieve high tempesature cooling. When working within the "cooling" range of cold/hot water, the molds are normally channeled to permit the flow of liquid at various controlled
I. (.
I*
Pipe Plug
Fro. 8.80. Typical cooling systems for injection molds.
437
,
s. Many tihes the channeling is set up for zone control in various molds, and in oving members, as well as the areas surrounding and plungen&ha?nels must be large enough to permit the of the cooling or heating medium. Figures 8.80, 8.81 and 8.82
Refer load 'and
'revision rr ~ i e mout of the mold 01: permits the ( flow into a v flows into the c ejector pin actu Cold slug are; systems should ejector pin, as d the pin below tl16 ,
440
PLASTICS MOLD ENGINEERING HANDBOOK
I
flow. Typical types of sprue pullers are shown throughout this chapter and in Fig. 8.84.<
The cavities of an injection mold may be formed by any of the conventional means, as discussed in; Chapter 3. Many molders prefer to have the cavities formed directly in o solid die bloek, or in w v m l blocks which are later fastened firmly on a suitable mounting plete. Tfih method of making a moldmay be objectionable from the tool-maker's sta+point because of difficulty experienced in the alignment of guide pins, dowel pins, etc. Other molders prefer to have the cavities inset as units in a retainer plate. This permits accurate boring of the holes designed to receive the cavity blocks when round sections am used. The retainer plates are often hardened to resist the abrasive action of the flowing material. In all cases. the sprues, mmtrs and gates should pass over hardened surfaces. FW some i s m p o h , this m a y not be too important, but friction will be reduced, b e c t ~mo1diag.s obtained, and less maintenance required if this pramdur~is followed. ~ h r i m plating e may be r e q e d i&some cases where the material has an^ acid cmteIyf W would attack aahary steels or unplated steels. The tool en@neer musf learn whethprr'
I
'
serts ~sed*ln?',lnjacflon .4; Molds
n place or they may be pressed in after molding. in, they should be held firmly in the mold. There cavity because of the speed with which the material are used, the gating of thk? mold should of material will not be pushed directly compound should have a swirling action if possible, spot. Inserts shoqid be located of material to enter is the hottest and the insert. If the brass color of inserts is .materials, the inserts may be oxidized by 1s will tend to magnify any internal I
Side cores may be considered as pait of the cavity, since they often afkc the position of the cavities in the mold. Adequate tapers, should always b provided'on the c o p pins. A taper of W 0 per side is considered satiskctory The minimum taper for side. h e r e are; of cour useful. Parts having ne from injection molds th setting material, the reason being the relative softness of the material 01 ejection. If possible, the core pins should be pulled before the mold is opened ant while the part is still in p a careful study.of the location of ejector pins with relation to the side core. Be sure the sequential operation provides for noninterference, otherwise expensive and repetitive be operated satisfactorily in many ways. Devices that are operated b machine are used, or the pins may be operated externally by electric ma operating through speed reducers. Solenoids, hydraulic or air levers, offset cams, angle guide pins or fmger cam are oom
-
441
E the core pins. When angle guide pins (Fig. 8.11A) are used, i offset bhould be less than 30°, with 2P a much safer figure. sqld'be not less t b n % in. ih diameter and should be well lubb&ide'long and trouble-free life. On machin& vrrZlich have large f&item, offset cams may extend from one half of the mold by half so that side cores or wedges may be operated in either
ice the molding, Side Cores
ION MOLDS F@R THERMOPLASTICS
I
inserts a; not often used in the injection process, a p short and it would take the operator longer to l a d the mold. Of course, if automath ,then it 1611 be necessary to use a loading y be opemted economically by ''.. _ tab inserts &auk4 place an ejector -?& w 7 b in~ the mold and diy ~e m.&t&dLcvn a
mia auto ma^* i
timing %&t apexring for e' tor ha8 &m;trd-tfie parts and k w b d tha next round of i n ~ e h ' ~ , .' ~' ., .-
y-
,
!$*
,:'w;:l".
7
a,,.
DESIGNING INJECTION MOLDS Standard Mold Frames
The standard frame was mentioned in Chapter 2 in the discussion pertaining construction of injection molds. Some additional information is given at the endsf this chapter and some commonly used sizes and other design are also praented. There are, of course, cases when it would be im: *
wlw
INd-ON
MOLDS FOR THERMOPLASTlCS
443
ard frame because of conditions surrounding some ecial feature of design. However, for many jobs, such as the ~ o w d e comr &own in Figs. 8.85 and 8.86, a standard frame could be used to good advantage. This mold requires a side-pull pin for a slot perforating one side of the lid. The pull-pin is operated by an offset cam fastened to the stationary d, the first thing the designer should do is to make a a master templet furnished by the manufacturer of the stanme. This master templet offers all the detail of the frame, but unnecessary to place the full detail in the layout. The only caution to rve is to make sure that temperature control channels, screw holes, c., do'not interfere with any of the frame dimensions. he compact is such that a body diameter for the plunger and . will allow for a %-in. land. An allowance of L/8 in. is made ter of 4% in. The diameter of the sprue brushugh the plunger plate, is 1 in. An allowance of % in. e bushing to the edge of the plunger, plus the 4% in. r distance of the cavities. when the mold is mounted 1runners and provides a more , the major portiori of the ontal. The side-pull pin will ,it is possible that the slide r the offset guide pin was sequently, it were closed
STANDARD MOLD FRAME
FIG.8.85. An injection mold for powder compact assemble4 in a standard frame. (See also Pig. 8.86.) (Courtesy Eastman Chemical Products, Inc., Kingsport, TN) .
sting. The offset cam is made in own in the rear elevation. This permits two socket-head ted 2% in. apart for-minimizing any deflection resulting in the screw holes. The slide is designed as a rectangular roper size hole for a sliding fit to the offset cam. The 30° bout the maximum that can be used if efficient operation If the angle is any greater, the guide pin will lend to move tion in which the pin moves instead of moving it at right on of the pin. The small tongue that molds the slot in made as a separate unit so that it can be replaced easily his also gives the heat treater a better chance to harden ince there will be no combination of thick and thin 10-32 screws are sufficient, as there will be little pull pose each other, that is, one cavity is on the ity is on the knockout side. The inside of the
444
PLASTICS MOLD ENGINEERING HANDBOOK
INJECTION MOLDS FOR THERMOPLASTICS
445
am of the compact (upper cavity in Section A-A) is more intricate
---shall &main on the ejector side. The top design is such that no ejector can be us& and this means that the runner, sprue and gate must be depended upon push the molding from the cavity as the sprue lock pin moves forwa rf v k h are needed.on this mold, they can be placed at the tongue in
i.
XNOCKOUT P/N
I
CLEARANCE a TOR AIECWAN/&U
to theBplmost liable to stick when the parts are ejected. Another reason for placin &em in this manner is to clear the 19/32 in. drilled hole in the hnrk-lln SWe. This hole must clear both the dowel pin holes in the frame [rmnce holes for the ejector pins. In & p a t i o n of the mold, the ejector half will move away from the rlf for a dista~keof $$ in. At this time, the offset guide pin
F
OFFSET G W PIN
since t h i s & & a m o u n w f f s e t in the guide pin. Continued movement salf will cause the ejector mechanism (which usually consists
B*-
SttXTJON THROUGH A -A
CAVITY
\R
e
,
TOP CLAW PLATE M
RlRULEL
of operati
versed as $he mold closes:
BOTTOM C L W PLATE
E BUSHIN6
W E T V WMS TOR RETURN PIM)'"
LOCAT I M RING
1.
S P M E LOCK PIN
smc€R WQ€ P/N &SHIN0
I, KNOCKOUT
GU/OE PIN
MR
KNvrCKOUT PIN HAT€
i l l t ~ ~ c C - d C ~ - H I C 8 SECTION THROUGH
C-C
FIG. 8.86. Cross sections of mold shown in Elg. 8.85. Section through A-A sham detail side-pull pin.
$earance hole should be not more than 1/32 in. larger @B if this construction is used. The best construction pia,hard&ed and insert a hardened bushing in the b#&el"lminate any side or end play from the assembly, ~ k s e will s be placed on the ejector pins. Soft supand may need replacing, and the holes in the for a fit to the support pins. qirlmst +ways located in the stationary half of the _ t h I they will not interfere with free ejection of the Wtic cycle islused. If the pins were put in the movable poasible for the shot of parts to hang on one r thp mold when it was closed for the next shot.
